FIG. 1 shows a conventional apparatus for conducting this type of measurment. In the drawing, a reference numeral 1 designates an object for examination; a numeral 2 refers to a light source; 3 denotes a power source for applying an electric potential to the light source 2; 4 represents a photoelectric multiplying tube; 5 a power source for applying an electric potential to the photoelectric multiplying tube 4; and 6 indicates a detector for measuring photo-current of the photoelectric multiplying tube 4.
Light emitted from the light source 2 passes through the object for examination 1 containing therein microorganism. The transmitted light is received by the photoelectric multiplying tube 4, and its intensity is measured by the detector 6 as a photocurrent value of the photoelectric multiplying tube 4. Since there is established a definite relationship between absorbance and concentration of microorganism existing in the abovementioned object for examination 1, thus obtained, when a visible light is used as the light source, the concentration of microorganism can be evaluated by measuring the absorbance of the object for examination. As the result of, or, in connection with, this, the cell counts can be evaluated.
Also, as an other method for measuring the microorganism activity, there has been known a method for optical measurement of a quantity of biological substance relative to the energy metabolism which is called "ATP" (Adenosine Triphosphate) or "NAD(P)H" (Nicotinamide Adenine Dinucleotide(phosphate)) contained in microorganism.
As mentioned in the preceding, the conventional method for measuring the cell counts or the activity of microorganism is to measure the absorbance of an object for examination 1, on account of which such method is effective only if the object for examination 1 is composed of one kind of microorganism and contains no foreign substance such as sludge, etc. However, if the object for examination 1 is composed of many kinds of microorganisms and, moreover, if foreign substances are contained within the examined object, it was impossible to selectively measure the cell counts or activity of a particular kind of microorganism. Further, since ATP and NAD(P)H are biological substances existing in all kinds of microorganisms, the method is not suitable for measuring the cell counts or methane producing activity of Methanogens alone.